Models for structural transitions in nitrates

A number of alkali and alkaline-earth nitrates crystallize in ionic structures related to the NaCl and fluorite crystal structures, respectively. This suggests that their cohesive properties may be usefully described by means of a phenomenological ionic model (Z. Akdeniz, M.P. Tosi, Zs. Naturforsch., 59a, 957 (2004)). In the present work we discuss from this viewpoint the structural transitions that take place in these materials. We first present a Bragg-Williams type approach to the orientational disordering of the NO3 groups and to positional melting in sodium nitrate. We then discuss the stability of the superionic CKN glass in terms of strongly bound Ca-2(NO3)(7) units surrounded by highly mobile potassium ions